Well known as CATV systems provide multi-channel television broadcasting utilizing a tree-type distribution network, for instance, shown in FIG. 1 or a star-type distribution network. Recently, these CATV systems are utilized for services transmitting information with high speed and capacity such as the Internet, etc., and data communication in digital broadcasting from terminals of digital broadcasting subscribers to the center station.
In such CATV systems, transmission lines such as coaxial mainlines, optical fiber cables, etc. are laid between the center station and terminals of the CATV system subscribers. Disposed in the transmission lines are CATV equipments such as bi-directional amplifiers for bi-directional CATV signals, and devices such as distributors, couplers, etc. for dividing CATV signals.
FIG. 2 is a block diagram showing one example of the use of CATV equipments in a CATV system.
In FIG. 2, bi-directional amplifiers 15a to 15c out of bi-directional amplifiers 15a to 15d are disposed in the mainline.
A branch cable downstream side of a protector 18 is led into a subscriber residence and connected to a computer 16, a television receiver 14, etc., via a bi-directional amplifier 15d, a noise filter 19 such as choke coils, etc. and a splitter 20. On the other hand, the line upstream side of the protector 18 is connected to the headend 10 via a distributor 13, a coupler 12 and the bi-directional amplifiers 15a to 15c. An RF modem connected to the computer 16 and a home terminal connected to the television receiver 14 are not shown in FIG. 2.
FIG. 3 is a block diagram showing one example of the bi-directional amplifier used in a CATV system. The bi-directional amplifier 150 comprises a forward amplifier, a return amplifier, a filter, and a coupler and/or a distributor. In FIG. 3, CATV signals input to an input terminal 160a enter a forward amplifier 180a, which amplifies the signals to the predetermined level, via a high-pass filter 170a and an attenuator 185a, and then the amplified signals are output to the output terminal 160b via a high-pass filter 170b and a coupler 190. Return signals traveling from a terminal of the CATV system subscriber input to a coupling terminal 160c, enter a return amplifier 180b, which amplifies the return signals to the predetermined level, via a coupler 190, a low-pass filter 175b and an attenuator 185b, and then the amplified signals are output to the input terminal 160a via a low-pass filter 175a. The coupling terminal 160c may be used as a monitor terminal to measure the output level. A distributor may be used instead of the coupler in the bi-directional amplifier.
FIG. 4 is a block diagram showing one example of a coupler 100 used in a CATV system. Forward or downstream signals are input to an input terminal 120a and output to an output terminal 120b and a coupling terminal 120c via a coupling transformer 110. Return signals input to the output terminal 120b are output to the input terminal 120a, not to the coupling terminal 120c. Return or upstream signals input to the coupling terminal 120c are output to the input terminal 120a, not to the output terminal 120b. 
The coupling transformer 110 comprises a first transformer 110a and a second transformer 110b. One example of the coupling transformer 110 is constituted by a multi-hole ferrite core with windings as shown in FIG. 6. The ferrite core shown in FIG. 6 comprises two through-holes, called “spectacle core.” A winding L1 of the first transformer 110a passing through one through-hole 55a is connected to the input terminal 120a at one end and to the output terminal 120b at the other end. A winding L4 of the second transformer 110b passing through the other through-hole 55b is connected to the terminal 120c at one end and to the resistor 105 at the other end. The winding L2 is wound around the through-hole 55a, while the winding L3 is wound around the through-hole 55b. One end of the winding L2 and one end of the winding L3 are connected to each other as a center tap and grounded. The other end of the winding L2 is connected between a winding L4 and a resistor 105 and grounded via the resistor 105. The other end of the winding L3 is connected to the output terminal 120b of the winding L1.
FIG. 5 is a block diagram showing one example of a distributor 200 used in a CATV system. This distributor 200 comprises an input terminal 220a connected to the headend side and output terminals 220b, 220c connected to the terminal side to bisect signals by a distributing transformer 210.
One example of the distributing transformer 210 is constituted by a toroidal core shown in FIG. 7 and windings L1, L2 wound around the toroidal core. The winding L1 has one end connected to the input terminal 220a and the other end connected to a ground. Both ends of the winding L2 are connected to output terminals 220b, 220c, with a resistor 205 connected in parallel with the winding L2. The middle points of the windings L1, L2 are connected to each other. With respect to the return signals, the input terminal 220a functions as an output terminal, while the output terminals 220b, 220c function as input terminals.
The turn ratios of the windings in the coupling transformer and the distributing transformer may be properly determined in accordance with coupling required for each transformer.
When the coupling transformer and/or the distributing transformer are used in the bi-directional amplifier, each terminal may properly be provided with a DC-blocking capacitor so that the electric power of the power supply overlapped to high frequency signals to supply electric power to the amplifiers is not applied to the above transformers.
In such a CATV network, data communications such as the Internet use a frequency bandwidth which television broadcasting does not use. In Japan, forward signals such as TV signals of VHS, UHF, BX, CS, etc. from a center station use a transmission bandwidth of 70 MHz to 1.3 GHz. The current CS digital broadcasting uses a frequency bandwidth up to 2.61 GHz. In bi-directional CATV networks, return signals such as data signals in the Internet, etc. from subscriber terminals to a center station use a transmission bandwidth of 10 MHz to 55 MHz. The U.S. and Europe use a frequency bandwidth of 5 MHz to 70 MHz for return signals (data signals), though there is slight difference in the frequency bandwidth, which television broadcasting does not use, from Japan.
The return signals received by the center station (headend) contain large noise components entering from subscribers, mainlines and transmission equipments of branch lines. Such noise called “ingress noise” deteriorates the quality of such as a C/N ratio, etc. of return signals and destabilizes the Internet connection. When large-volume files and video data, etc. are transmitted, the ingress noise causes serious problems, requiring proper measures.
It has conventionally been considered that the ingress noise is mainly caused by thermal noise generated in terminal equipments such as computers, bi-directional amplifiers, etc., the noises of domestic electric appliances (for instance, the ignition noise of hairdryers and fluorescent lamps, pulse noise generated at the time of switching microwave ovens, the compressors of inverter air conditioners and refrigerators, etc.), citizens' radio communications, short-wave broadcasting, etc. The measures for such ingress noise have conventionally been as follows:    (a) Increasing the shielding of CATV circuits and equipments.    (b) Cutting off signals in a return bandwidth in the terminals using no return signals.    (c) Cutting off branching circuits generating large ingress noise.    (d) Using an HFC (hybrid fiber and coaxial) system comprising optical fibers in mainlines and photoelectric converters disposed at the ends of mainlines for connecting branch lines constituted by coaxial cables to subscribers, to decrease the number of terminals connected to one tree-type transmission system to reduce noise to some extent.
There are also the following measures investigated by various companies, though some measures fail to utilize the existing infrastructure. Accordingly, they are not put in practical use.    (e) Changing the frequency bandwidth of return signals to that causing little noise.    (f) Using a modulation system highly resistant to noise.    (g) Using only one terminal in a return path, from which LAN is constituted by using LAN cables, telephone lines, etc.
Though the above measures provide some effects, it has been found that return signals contain other noises than the above noises, which appear to be generated due to the differences of carrier waves of channels contained in forward signals. As described above, the forward signals use a transmission bandwidth of, for example, 70 MHz to 2.61 GHz, and CATV channels are divided to an interval of 1 to 8 MHz. Noise components substantially equal to this frequency interval are generated in the entire frequency bandwidth of return signals.
Search on the cause of generating noise components has now revealed that once ferrite cores constituting transformers used in CATV equipments such as couplers and distributors are magnetized, the ingress noise increases.
The ferrite cores used in the CATV equipments are usually made of ferrite materials that are not magnetically saturated by electric power used in the CATV systems. However, when a surge current is caused to flow in the CATV equipments by lightning, etc., the ferrite cores are likely to be magnetically saturated. Accordingly, a surge current is prevented from flowing into transformers by connecting a capacitor to each terminal of a coupler, and further by connecting a choke coil between the capacitor and the ground to achieve DC or low-frequency connection to the ground (JP 2001-285819 A), or by disposing a high-pass filter for removing a lower frequency bandwidth than the lowest frequency of the CATV signal (JP 2002-204439 A).
In the conventional method, the magnetic saturation of ferrite cores is prevented by removing a surge current flowing into coupling transformers and distributing transformers by choke coils, filters, etc. This method, however, needs circuit elements such as inductors, capacitors, etc. for constituting choke coils and high-pass filters, resulting in increase in the size of CATV equipments and the number of assembling steps to increase cost. As a result, the assembling of the bi-directional CATV system becomes costly. In addition to the magnetization by a surge current, the ferrite cores are likely to be magnetized by nearing permanent magnets to the ferrite cores, or by unintentionally applying an external magnetic field to the ferrite cores by magnetized tweezers, etc.
Accordingly, it is desired that ferrite cores per se provide simpler measures without needing the above-described complicated means. However, it has been considered difficult to overcome the ingress noise generated by the magnetization of the ferrite cores.